The present invention relates to an apparatus and method for controlling an electromagnetic valve that opens and closes a fuel passage.
A compressed natural gas (CNG) vehicle uses natural gas as fuel. The fuel gas is mixed with air and burned to obtain power. A CNG vehicle is provided with a fuel tank that is charged with pressurized fuel gas. The fuel is sent to the vehicle engine through a fuel passage. The fuel tank is provided with a valve to open and close the fuel passage. A manual type valve, which has a handle to manually move a valve body and open or close the fuel passage, is often employed in CNG vehicles. However, recently, an electromagnetic type valve, which uses electromagnetic force to open and close the fuel passage, has been widely used. Japanese Unexamined Patent Publication No. 6-241341 describes a typical electromagnetic type valve. In the electromagnetic valve, which includes a valve body for opening and closing the fuel passage, a spring and the gas pressure in the tank urges the valve body in a direction closing the fuel passage. An electromagnetic solenoid moves the valve body in a direction opening the fuel passage when actuated.
In a CNG vehicle that employs the electromagnetic type valve, a predetermined amount of electric current flows through the electromagnetic solenoid when the engine is started. The predetermined amount of current is set at a value that moves the valve body and opens the fuel passage even if the difference between the pressure inside the fuel tank and the pressure outside the fuel tank is high. The actuation of the solenoid moves the valve body against the force of a spring and the gas pressure in the tank. This permits the natural gas in the fuel tank to be supplied to the engine through the fuel passage. The electromagnetic solenoid is de-actuated when the engine is stopped. This causes the force of the spring and the force resulting from the pressure of the natural gas to move the valve body in a direction that closes the fuel passage. A rubber seal, which is attached to the valve body to enhance the sealing of the valve, abuts against a seat of the valve. This closes the fuel passage and stops the supply of natural gas to the engine.
It is required that the electromagnetic valve be provided with a leakage prevention function that prevents the fuel from leaking out into the atmosphere when a fuel passage component is punctured or damaged. To fulfill this requirement, in the prior art, a mechanical leakage prevention valve is provided in the fuel pipe. An example of a prior art leakage prevention valve is shown in FIG. 14. The prior art leakage prevention valve is attached to the upstream end of an electromagnetic type valve (not shown). The leakage prevention valve includes a case 100, a valve body 101, and a compression spring 102. The case 100 has a plurality of apertures 100a that permit fuel to enter the case 100 from the tank (not shown) and to enter the tank through the case 100 when fuel is charged from a fuel fill line (not shown). The valve body 101 has a plurality of communication holes 101a and an outlet sleeve 101b. The compression spring 102 urges the valve body 101 in a direction that opens the communication holes 101a. In this state, the fuel tank is constantly communicated with a fuel passage (conduit 26) through the communication holes 101a, the apertures 100a, and the outlet sleeve 101b.
The action of the valve body 101 differs when the fuel passage is in a normal state (no damage to the fuel passage component) and when the fuel passage is in an abnormal state (damage existing). In the normal state, the valve body 101 opens the communication holes 101a. In the abnormal state, the flow rate of the fuel increases. Thus, the difference between the pressure in the fuel tank and the pressure in the fuel passage (conduit 26) increases. The pressure difference closes the communication holes 101a by causing the valve body 101 to enter the conduit 26. However, the urging force of the spring 102 is strong enough to keep the communication holes 101a opened during the normal state, even if the pressure difference or the flow rate of the fuel reaches a level that is a maximum for the normal state. In the abnormal state, the pressure difference increases beyond the maximum pressure difference for the normal state. The urging force of the spring 102 is not strong enough to keep the communication holes 101a opened when the pressure difference reaches such a high value. As a result, the communication holes 101a are closed.
In the prior art electromagnetic valve, to positively move the valve body to a position that opens the fuel passage and to maintain the valve body at that position, even when the pressure difference between the fuel tank and the fuel passage is high, a large relatively high current must flow through the electromagnetic valve. In other words, a relatively high current must flow through the electromagnetic valve regardless of the actual pressure difference. As a result, if the pressure difference is smaller than the maximum value (which is usually the cases), an unnecessary amount of current flows through the electromagnetic valve and consumes electric power in a wasteful manner. In addition, the actuation of the electromagnetic solenoid generates a large amount of heat. The heat raises the temperature of the components arranged about the valve body. Thus, the rubber seal is heated to a high temperature. This may deteriorate the rubber seal and degrade the durability of the seal.
Furthermore, when employing the prior art mechanical leakage prevention valve, the prevention valve must be installed on the electromagnetic valve for complete leakage prevention of the fuel passage. This results in a fuel supply system having a complicated structure. In addition, the prevention valve uses a spring provided with a relatively strong urging force. Thus, the valve body does not close the fuel passage unless the pressure difference becomes very high. In other words, the valve body does not move immediately unless the leakage and the pressure difference becomes very high. Additionally, when servicing the fuel passage or other parts in the fuel supply system, the pressure in the pipe is lowered to atmospheric pressure. Thus, servicing may lead to erroneous functioning of the leakage prevention valve.